Through the years there have been quite some examples of former MLB players that kept on playing baseball, although on a lower level. The most recent example is Bartolo Colon. After a 21-year career in the Bigs, he has signed a deal with a team in the Mexican League.
Previous examples of players that could not stop playing are Julio Franco, who kept playing until his 50s, Rickey Henderson and Manny Ramirez, who might be playing in Italy in the upcoming season.
Julio Franco kept playing, with some intervals, until his 50s. He spent time in the Majors, the NPB, the KBO, and several obscure leagues in the USA and Japan in a career that lasted more than 25 years. In 2004, he passed Cap Anson as the oldest regular position player in the Majors.
Rickey Henderson was another fine example. After a 25-year Major League career, the All-Time stolen base record holder kept on playing in the Atlantic League (independent), the Golden Baseball League (independent). The San Diego Surf Dawgs won the league’s championship in their and the league’s inaugural year (2005). This would be his final professional season.
Manny Ramirez also had a lengthy career. After 19 years in the Majors, he kept on playing in the Pacific Coast League (AAA), the CPBL (Taiwan) and the Shikoku Island League (Japan, independent).
The newest example as written in the prologue is Bartolo Colon. The hefty pitcher kept on playing in the Majors for 21 years. In 2019 he did not play at all as he was not signed by an MLB team. But… the other day the news was announced that Big Sexy has signed a one-year deal with the Monclova Acereros of the Mexican League. Even though this league is not associated to the Major Leagues, it is regarded as an AAA league.
By joining the Acereros, Colon will be part of the reigning Mexican League champion.
The 47-year old Colon did not have an ERA below 5.78 since 2016. A successful stint with the Acereros may draw some attention from MLB clubs.